This invention relates generally to gas turbine engines and, more particularly, to an apparatus for minimizing rotor/shroud and stator/rotor clearance during steady-state and transient operation.
As turbine engines continue to become more reliable and efficient by changes in methods, designs and materials, losses which occur from excessive clearances between rotors/shrouds and stator/rotor become more important in the many design considerations. Originally, the primary efforts in regard to clearance control were directed to the turbine/shroud relationship, whereas recently these considerations are being given to control of the compressor rotor/shroud and stator/rotor relationship.
In many turbine engine applications, there is a requirement to operate at various steady-state speeds and to transit between these speeds as desired in the regular course of operation. For example, in a jet engine of the type used to to power aircraft, it is necessary that the operator be able to transit to a desired speed whenever he chooses. The resulting temperature and rotor speed changes bring about attendant relative growth between the rotor and the surrounding shroud/stator and, in order to maintain the desired efficiency, this relative growth must be controlled. The object is to maintain a minimum clearance between the stator and rotor while preventing any interference therebetween which would cause rubbing and resultant increase in radial clearance during subsequent operation. When considering the transient operating requirements, as mentioned hereinabove, the relative mechanical and thermal growth patterns between the rotor and the shroud present a very difficult problem. If the system were to operate only under steady-state conditions, it would be a relatively simple matter to establish the desired close clearance relationship between the rotor and the stator to obtain the greatest possible efficiency without allowing frictional interference between the elements. However, in order to accommodate the transient operation requirement, the engine is generally designed so as to have adequate clearance during the most extreme relative growth operating condition; usually for hot rotor rebursts. Thus, during other operating conditions, including that of the cruise where the engine running time is generally the greatest, the clearance between the components can be greater than the minimum clearance desired for maximum efficiency.
One method of minimizing the tip clearance of turbomachines has been to properly select the various materials which exhibit thermal properties that will assist in matching the radial responses of the rotor and shroud at different engine operating conditions. Thus, the thermal coefficient of the shroud material or that of the shroud support material is a very important design consideration. However, that alone is not sufficient to provide for adequate clearance control.
Another approach has been to flow cooling air over the shroud structure or the shroud support structure in order to better match the thermal growth patterns of the rotor. Provision has even been made to vary the temperature or the flow rate of the cooling air as, for example, by the use of compressor air whose flow or temperature may naturally vary with the changes in speed of the engine. Such a passive system does provide improved clearance characteristics but may still be inadequate for attaining best possible efficiency.
It is, therefore, an object of this invention to provide a turbomachine which operates at increased overall efficiency and performance levels.
Another object of this invention is the provision for controlling the clearance between rotor/shroud and stator/rotor components of a turbomachine.
Yet another object of this invention is the provision for minimizing clearance between a rotor and shroud during both transient and steady-state operation.
Still another object of this invention is the provision for a clearance control system which is effective in use and economical in operation.
Yet another object of this invention is the provision for setting optimum clearances for such conditions, as engine starting, and during periods of operation, such as sea level takeoffs when larger clearances are needed for expected high maneuver loads and engine rotor-to-stator relative deflection.
These objects and other features and advantages become more readily apparent from reference to the following description when taken in conjunction with the appended drawings.